February 8, 1998

By MARILYN STASIO

killer named Keller makes his mark in Lawrence Block's HIT MAN (Morrow, $22), a collection of stories (all but one previously published) that reads like the flowing memoir of a man who is letting it all out for the first time. The thing about Keller is that he's the top of his class -- an ace assassin with steady hands, cool nerves and perfectly honed professional skills. The other thing about Keller is that he's having a major midcareer crisis. By job definition a loner, who can't acquire a girlfriend, a therapist or even a dog without being prepared for the (often dire) consequences, the guy really needs to think and talk. Hence, the ironic tone and confessional content that make these intimate tales so funny and full of rue.

Each story, or ''chapter,'' as designated here, presents Keller with an unusual challenge (conflicting assignments, an impregnable target, a catastrophic piece of misdirection), along with a fresh wave of existential angst. In two of the best stories, ''Keller on Horseback'' and ''Keller on the Spot,'' he becomes involved in the lives of his targets and is forced to make moral judgments. Accepting his fate, he hauls himself across the country, eating at cheerless dives and drinking alone while fantasizing about retiring, or at least taking up a hobby. The man may have no ethics (''Ethics? What do I know about ethics?''), but he definitely has a way about him.

How much crime could there be in Blue Deer, Mont.? With a population of 5,000 inbred, deeply suspicious souls, the eccentric little burg where Jamie Harrison sets her regional mysteries is no place for secrets. That's why Sheriff Jules Clement, the pensive hero of this offbeat series, goes into one of his soulful depressions when a virtual crime wave hits town in AN UNFORTUNATE PRAIRIE OCCURRENCE (Hyperion, $22.95).

Since everyone in Blue Deer is related (by blood, marriage, business, friendship, hatred or hanky-panky) to everyone else, Jules knows that ''if he played his cards right, he could offend everyone in town.'' He will probably have to arrest one of his own relatives for the murder of a local rancher, and some mother's son will no doubt prove to be the serial rapist who has just claimed his fifth victim. Meanwhile, Jules has already alienated most of the town elders by questioning them about the old skeleton that a hunter dug up on an island in the Yellowstone River.

Harrison takes her time resolving these criminal matters, allowing us to linger in Blue Deer long enough to learn its history, drink in the scenery and laugh at the kinks and quirks of its idiosyncratic residents. No wonder the world-weary Jules came running back home the first chance he got -- the place is heaven.

Call me a kvetch, but I can't buy the premise that a New York City police detective (not even ''the N.Y.P.D.'s most famous detective'') would be traipsing around Iceland, Belfast and Dublin on an international political case involving I.R.A. terrorist bombings and the assassination of the British Foreign Secretary. Dan Mahoney offers a rationale, in ONCE IN, NEVER OUT (St. Martin's, $24.95), for Detective Brian McKenna's extraordinary assignment. Officially, he is sent to Reykjavik to investigate the murder of Meaghan Maher, the sister of an aide to the Cardinal Archbishop of New York, who ran afoul of an I.R.A. bomber while changing planes in Iceland. It's a stretch.

But if McKenna looks awkward having a heart-to-heart with an Irish Cabinet minister, he's wholly believable when he pitches in to help his Icelandic counterpart, Thor Erikson, on the detailed forensic procedures of working the murder case. Mahoney, a retired police captain with 25 years' experience, knows the drill on conducting a manhunt for the ''total psychopathic sexual sadist'' who made mush of Meaghan, and he takes us through the crime scene photos, the morgue visits, the criminal profiling, the computer searches and all the rest of it with scientific precision and absolute authenticity.

Most actors don't really mean it when they say they would ''kill'' for a juicy part. Philip Fletcher does. The English character actor, whose unorthodox career moves are fondly chronicled in Simon Shaw's poisonously funny theater mysteries, comes up against a blackmailer who knows of his murderous history in THE COMPANY OF KNAVES (Thomas Dunne/ St. Martin's, $22.95). Always inventive, Philip outdoes himself when he goes undercover as Marlene von Trapp, a transvestite dominatrix (''Decadent, darlink, iss my middle name'') whose whip-snapping rendition of ''Alabama Song'' wins Philip a gig at the cabaret where the blackmailer has hidden the damning evidence. The ensuing mischief and mayhem are predictable, but Philip never is. After making a careful study of Marlene (''Was he a man pretending to be a woman, but knowing full well that he wasn't? Or was the illusion meant to be more than skin-deep?''), he creates a memorable character and gives the performance of his life.

Dorothy Simpson's village mysteries, although dry and overly genteel, are the very model of the methodically constructed detective story. In ONCE TOO OFTEN (Scribner, $21), the 14th book in her Inspector Luke Thanet series, she uses her plot-driven formula seamlessly and makes it look effortless. An unlikable woman named Jessica Dander, a reporter for a newspaper in Kent, is found lying at the foot of the stairs in her home, her neck broken. Even though the death appears to be an accident, any number of people might have killed her: the husband she humiliated, the lover she annoyed, the teen-age admirer she fascinated. With the exception of Thanet, a thoughtful man with a rich emotional history, the characters are well observed without being especially complex. (Simpson's peculiar forte seems to be the behavioral quirks of long-married couples.) They function flawlessly, however, dropping each piece of evidence into place and pushing the story to its artful ending.